Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Although surface coatings of excellent quality can be achieved by means of cationic electrodeposition, a problem associated with this means of coating is the development of surface defects upon curing, particularly craters. The cause of such surface defects can be a result of the very nature of the components of the electrocoating composition, that is, causes inherent in the system. Typically, however, the cause of such surface defects is the impurities that are carried into the electrocoating bath with the workpiece. Examples of such impurities can include lubricating oil, anti-corrosion grease, joint sealing compounds and the like.
As the electrocoating composition is deposited onto the conductive substrate, the impurities are carried along with the coating composition and are deposited as well. When the coated substrate is cured, craters are formed due to incompatibility between the impurity and the resinous phase of the electrocoating composition.
The use of silicone additives such as SILWET L-7602, a silicone oil commercially available from OSi Specialties, a subsidiary of Witco Corporation, to prevent crater formation is well known in the art. Although these silicone additives are quite effective for crater control in electrocoating compositions, they also pose the serious disadvantage of intercoat adhesion failure of subsequently applied coating layers.
Japanese Patent Application J61,115,974 discloses the addition of a reaction product formed from a polyepoxide resin modified with dimeric fatty acids and a polyoxyalkylene polyamine to cationic electrocoating compositions to suppress crater formation. These products, however, can result in intercoat adhesion failure of subsequently applied primers and/or topcoats. U.S. Pat. No. 4,432,850 discloses the addition of a reaction product of a polyepoxide resin with a polyoxylene-polyamine containing primary amino groups to cationic electrocoating compositions to eliminate or minimize crater formation. However, to achieve maximum effectiveness as a crater control additive, this polyepoxide-polyoxyalkylenepolyamine resin sometimes must be added at quite high levels (i.e., 0.5 to 40 percent by weight), thereby diluting the properties of the other film-forming cationic resins which are present in the electrodepositable coating composition.
U.S. Pat. Nos. 5,427,661 and 5,501,779, both to Geist et al., disclose the addition of a homopolymer or copolymer of an alkyl vinyl ether to cationic electrocoating compositions to suppress the occurrence of craters in the deposited surface coating materials. Such alkyl vinyl ether materials provide adequate crater control while overcoming the intercoat adhesion failure observed with the above-mentioned crater control additives. However, these alkyl vinyl ether materials can impart the tendency in a coating to telegraph solvent wipe marks and negatively affect wettability of a subsequently applied primer or topcoat.
Copending patent application Ser. No. 08/986,812 filed Dec. 8, 1997, now U.S. Pat No. 6,033,545, discloses the addition to electrocoating compositions of a polysiloxane obtained from the reaction of a polysiloxane containing silicon hydride with a material containing one or more unsaturated bonds to suppress crater formation.
U.S. Pat. No. 5,089,101 to Hayashi et al. discloses cationic electrodepositable coating compositions comprised of a neutralization product or a quaternary ammonium salt of a comb-shaped copolymer of an ethylenically unsaturated monomer having a hydrocarbon chain with at least 8 carbon atoms at the molecular ends, a cationic (meth)acrylic monomer, 1-vinyl-2-pyrrolidone and an ethylenically unsaturated monomer different than those previously mentioned, a cationic epoxy resin and a pigment. The copolymer is characterized by relatively oleophilic long side-chains which are bonded indirectly to the main chain containing a hydrophilic nitrogen containing (meth)acrylic monomer. This comb-shaped structure provides a resin with excellent pigment dispersion properties and a composition with excellent weatherability and corrosion resistance.
The prior art references do not teach the use of either polyvinylpyrrolidone or vinyl pyrrolidone copolymers as components in electrodepositable coating compositions to provide improved crater control.
It would, therefore, be advantageous to provide an electrocoating compositions which suppresses or eliminates the occurrence of surface defects such as craters, without adversely affecting intercoat adhesion of primers and/or topcoats which may be subsequently applied to the deposited coating material.